We propose to develop an electrical stimulation system with adaptive capabilities for exercise and rehabilitation. The target populations are: 1) individuals with complete or incomplete SCI who are interested in improving their physical fitness through electrically stimulated lower-extremity exercise, 2) individuals with complete or incomplete SCI who seek to develop or maintain adequate muscle strength to allow routine use of an FES system for standing, and 3) individuals with incomplete spinal cord injury, brain injury, or stroke who have the potential to regain voluntary lower extremity motor function through motor retraining therapy. The proposed portable exercise unit will include sensors and an adaptive control algorithm to provide repeatable movements. We anticipate that the adaptive controller will provide safe and effective exercise and that the adaptive controller will allow the exercise goals to be defined in movement rather than stimulation-specific parameters. For motor retraining, the system will encourage maximum voluntary effort by providing only the stimulation required to supplement the user's effort. The Phase I aims were achieved successfully. Phase I results clearly demonstrate the feasibility of our approach and have laid a strong foundation for the proposed Phase II effort. In Phase II, we will: 1) develop sensors to provide direct knee angle measurements and simplify donning and calibration, 2) develop a prototype adaptive stimulator for use by individuals with complete or incomplete SCI, 3) develop a clinical software suite for protocol programming, performance evaluation, and documentation, 4) assess the efficacy of the device in a study involving individuals with complete injuries who wish to increase muscle mass, strength and endurance through a long-term electrical stimulation based training protocol, and 5) assess the efficacy of the device in a study involving individuals with incomplete injuries who have some degree of voluntary control over the target muscles and who wish to improve muscle condition and potentially improve their ability to perform functional voluntary activities.